Light emitting diodes (LEDs) are semiconductor devices, which generally comprise a semiconductor stack having a p-n junction that is often referred to as the active region. The p-n junction of the semiconductor stack includes p-type and n-type semiconductor layers stacked together in close contact. LEDs are capable of emitting a relatively narrow-spectrum light when electrically biased in the forward direction of the p-n junction. The light generated by LEDs is emitted spontaneously upon the application of the electric voltage to the p-n junction, as opposed to the stimulated emission of lasers and laser diodes, which rely on stimulated light reflected within, and released from, a resonant cavity.
LEDs have been used as photonic interconnects to communicate data. In such applications, an LED is modulated by varying the intensity of the light produced by the LED and/or by turning the LED on and off to represent data as “0's” and “1's.” However, conventional LEDs cannot be modulated at desired speeds, because the photon lifetime of the generated light is relatively long. Photon lifetime refers to a time constant that describes the decay (or the growth) of the number of photons, such as the amplitude of the light output. LEDs can be modulated at most to 1 GHz and lasers can be modulated to 40 GHz or more. Thus, the photon lifetime for LEDs is about 1 nanosecond versus tens of picoseconds for semiconductor lasers. Consequently, the utility of LEDs in the field of photonic interconnects is limited by the inability to achieve high speed modulation. In addition, increasing the modulation of an LED to 1 GHz greatly reduces its light output.
In contrast to LEDs, lasers have the ability to generate light with a shorter photon lifetime. Thus, lasers can be modulated at higher rates. For example, as described above, some lasers can be modulated to 40 GHz or more. Lasers, however, lack many of the beneficial attributes inherent to LEDs, such as reduced manufacturing costs, insensitivity to temperature variations, and insensitive to reflections. Reflections back into an LED will not de-stabilize its output, whereas in the case of a laser, reflections will cause instability in its light output. Attempts to reduce the photon lifetime of LEDs by introducing quantum wells into the active regions and doping have met with varying degrees of success. Current techniques still do not allow LEDs to be modulated at levels similar to that of a laser.